Examples of a method of hardening a metal surface are a chemical surface hardening method in which a metal surface is hardened by changing a chemical composition at the metal surface and a physical surface hardening method in which a metal surface is hardened by performing only a heat treatment without any change in a chemical composition at the metal surface. Examples of a chemical surface hardening method are cementation, nitriding, sulfurising, and boreding, and examples of a physical surface hardening method include annealing. The surface hardening methods are used to improve abrasion-resistance, fatigue strength, corrosion-resistance, and seizure-resistance, and since, recently, a demand for durable, high-performance, and very lightweight machine parts, molds, and tools is increasing, engineers engaged in various fields are paying more attention to surface hardening methods.
Among the surface hardening methods, nitriding is a method in which an active nitrogen (also referred to as generator nitrogen) atom is diffused among metal, thereby performing nitriding. In the case of metal formed of a monometal or an alloy, rigidity, hardness, and abrasion-resistance of the metal are improved by, in general, solid-solution hardening by introducing nitrogen or carbon to the metal. The nitriding treatment may be performed by, in general, ion implantation or plasma deposition. A plasma nitriding is a method in which a vacuum furnace in a rarefied atmosphere of 1 to 10 Torr, hundreds of direct voltage V is applied to N2 gas and H2 gas to cause a glow discharge while a member to be treated acts as negative electrode (−) and a wall of the furnace acts as a positive electrode (+), thereby generating N+ and H+ ions. The N+ and H+ ions collide with a high kinetic energy with a surface of the member to increase the temperature to a treatment temperature and to perform nitriding.
However, the ion implantation nitriding or the plasma deposition nitriding require expensive equipment and high production costs. In addition, it is difficult to form a thick coating layer. Also, there is a need for nitriding aluminum (Al), titanium (Ti), or an alloy thereof without use of vacuum equipment.